On the Tertiary Iron Ore Measures. 163 



red and tlie bottom one dark-brown ; about three fathoms to the 

 east of the dyke the top seam is cut out, while the bottom one 

 increases in thickness. 



To the following I would draw special attention in the 

 GlenarifF district, I have never found the ore of the same quality 

 and quantity on both sides of a dyke, and from what I can 

 learn the same thing occurs in other mines ; as a rule a good 

 seam occurs on one side only which is generally on the east side. 

 To me this appears important as we find a similar phenomenon in 

 standing mineral veins, when the elvans or cross courses appeared 

 to act as a stop to the filling material. Thus in the case 

 of the pisolitic ore seam, it would appear that the dykes 

 which stop at the roof acted as a sort of stop for the material 

 which constitute the ore seam. As previously stated the pisolitic 

 ore is neither baked nor displaced by the majority of dykes 

 which stop at the roof ; while nearly invariably it is displaced 

 and indurated by the dykes which penetrate the roof, from which 

 it would appear, that the pisolitic ore was formed prior to the 

 latter, and subsequent to the former. Yet as previously stated, 

 the relation between the iron ore and the lignite, would suggest 

 the accumulation to be lacustrine. But on the other hand, 

 the pisolitic iron seam is not of even thickness, and is often 

 absent over large areas, also its thickening at one side of a dyke 

 and not at the other ; the pisolitic structure being well developed 

 in one place and scarcely discernable in another, and the largest 

 pisolites being always found next the roof decreasing both in 

 size and number as we descend from it, are facts difficult to 

 explain in a lake deposit. As however, none of the eminent 

 authorities who have written on these horizontal seams, have 

 put forward a theory that will satisfactorily account for those 

 peculiarities in their accumulation, it would be presumption in 

 me to do so. 



Notes added in the Press. 



No. 1. 



A large plan and section of the Iron Ore Measures was exhibited when 

 this paper was read ; also specimens of tlie various rocks and minerals 

 referred to in the paper, inchiding those showing the division between 

 the first or pisolitic, and the second or aluminous ore, and between the 

 latter and the pavement. 



